1. Technical Field
The present invention relates to a semiconductor light-emitting device, such as a light emitting diode (LED), and particularly to a technique for improving light-extracting efficiency.
2. Description of the Related Art
In a light emitting diode (LED) made of an AlGaInP-based material, the band gap of a GaAs substrate used for crystal growth is smaller than the band gap of a light-emitting layer. In this case, all the light emitted from the light-emitting layer cannot be extracted, that is, part of the light directed toward a light-extracting surface can be extracted, whereas the light directed toward the GaAs substrate is absorbed therein.
Japanese Laid-open Patent Publication No. 2002-217450 (hereinafter also referred to as Patent Document 1) discloses an LED manufactured by forming a semiconductor film made of an AlGaInP-based material on a GaAs substrate, which is a substrate for crystal growth (or growth substrate), attaching the semiconductor film to a support substrate via a reflective film made of a high-reflectance metal, and then removing the GaAs substrate. According to such an LED configuration that has no substrate for crystal growth, the light emitted from the light-emitting layer is not absorbed in the GaAs substrate but the light directed away from the light-extracting surface (i.e., toward the support substrate) is reflected off the reflective film, whereby the light-extracting efficiency is improved in the so-configured LED as compared with that in the related art.
However, light incident on the interface between the semiconductor film and a surrounding medium, such as air or a resin, at an angle of incidence greater than or equal to a critical angle will be completely reflected by total reflection and hence will not be extracted from the semiconductor film. The light that has not been extracted from the semiconductor film is repeatedly reflected therein; i.e., is reflected multiple times. The intensity of the light propagating in the semiconductor film decreases exponentially with the propagation distance (or optical path length). Specifically, the light reflected multiple times in the semiconductor film is absorbed therein (self-absorbed), and it is therefore difficult to extract the light out of the semiconductor film. For example, when an AlGaInP-based semiconductor film, which has a refractive index of 3.3, is encapsulated in a resin having a refractive index of 1.5, the critical angle is 27° and the reflectance at the interface between the semiconductor film and the resin is about 15%. In this case, the light that can be extracted from the semiconductor film is limited to about 4.5%.
Japanese Laid-open Patent Publication No. 2008-103627 (hereinafter also referred to as Patent Document 2) discloses a semiconductor light-emitting element having projections and depressions formed on a light-extracting surface of a semiconductor film. In the thus configured semiconductor light-emitting element, the light directed toward the light-extracting surface is scattered and diffracted by the projections and depressions, and the amount of reflection light due to total reflection at the interface between the light-extracting surface and the surrounding medium can be reduced, whereby the light-extracting efficiency can be improved. Patent Document 2 further describes that the projections and depressions are formed not to make the semiconductor film thin. The reason for this is that when the semiconductor film is thin, the series resistance thereof becomes high and the current therethrough spreads or diffuses insufficiently. Specifically, when the current spreads insufficiently in the semiconductor film, current density increases only in part of the semiconductor film. When the current density in the semiconductor film becomes higher than a threshold, carriers injected into the light-emitting layer overflow, resulting in decrease in the number of carriers that contribute to light emission and hence decrease in the light emitting efficiency. It is therefore necessary to spread the current over a wide area in the semiconductor film by making the semiconductor film thicker than a certain value.
Forming projections and depressions on the light-extracting surface side allows more light to be extracted from the semiconductor film, but the light reflected multiple times in the semiconductor film still exists. As described above, to encourage the spread of current in the semiconductor film, it is necessary to make the semiconductor film thicker than a certain value, but a thicker semiconductor film increases the propagation distance (or optical path length) of the light reflected multiple times in the semiconductor film and hence the amount of self-absorption increases, resulting in decrease in the light-extracting efficiency. Specifically, it has been difficult to reduce the amount of self-absorption of the light propagating in the semiconductor film without hampering or obstructing the spread of the current in the semiconductor film.
The present invention has been contrived in view of the circumstances described above. An object of the invention is to provide a semiconductor light-emitting device capable of minimizing the amount of self-absorption of light propagating in a semiconductor film without hampering or obstructing the spread of current in the semiconductor film.